1. Field of the Invention
The present invention relates to a frequency variation compensating method of a video receiving apparatus which provides for clear reception of a television broadcasting signal by compensating a local oscillating frequency of local oscillator during reception of a television broadcasting signal of predetermined channel by a video receiving apparatus.
2. Description of the Prior Art
A video receiving apparatus, such as a television receiver capable of receiving a television broadcasting signal or a video cassette tape recorder, includes a tuner for receiving a television broadcasting signal of a channel desired by a user.
The tuner section of the video receiving apparatus amplifies a signal received by an antenna at radio frequency amplifying section, variably changes an oscillating frequency of local oscillating section in accordance with the frequency of the television broadcasting signal of a channel to be received, and mixes the local oscillation signal of the local oscillating section to an output signal of the radio frequency amplifying section, thereby outputting an intermediate frequency signal.
The local oscillating section has to be exactly varied in oscillating frequency in response to a frequency of a television broadcasting signal of the channel desired to be received by a user, so that a television broadcasting signal of the desired channel is clearly received such that a clear picture without noise is displayed on a screen, and a voice signal can be output.
Therefore, the video receiving apparatus controls the local oscillating section to vary the oscillating frequency in response to the frequency of a received television broadcasting signal. An automatic fine tuning circuit is included in the video receiving apparatus so that an oscillating frequency of the local oscillating section can be constantly maintained.
The fine tuning circuit varies the level of a voice intermediate frequency signal in response to the oscillating frequency of the local oscillating section and converts the voice intermediate frequency signal of the received television broadcasting signal to a direct current. Therefore, the oscillating frequency is controlled by controlling the local oscillating section in response to the level of the direct current voltage. That is, the automatic fine tuning circuit varies the oscillating frequency of the local oscillating section, and determines the level of the direct current voltage of the voice intermediate frequency signal in response to the varying oscillating frequency of the local oscillating section, and the oscillating section is oscillated by an oscillating frequency becoming more than a predetermined level in a level of the direct current voltage of a discriminated voice intermediate frequency signal.
The automatic fine tuning circuit can exactly receive a television broadcasting signal by automatically controlling the oscillating frequency of the local oscillating section in response to a change of surrounding ambient temperature and humidity or various environmental changes when the video receiving apparatus receives a television broadcasting signal of a predetermined channel.
However, the automatic fine tuning circuit is operative only at an initial time of receiving the television broadcasting signal of the predetermined channel. The automatic fine tuning circuit cannot operate during reception of the television broadcasting signal.
That is, the automatic fine tuning circuit is operated each time the frequency of the television broadcasting signal is varied during channel selection or each time the oscillating frequency of the local oscillating section is varied, to control the oscillating frequency of the local oscillating section.
Since the oscillating frequency of the local oscillating section cannot be controlled by operating the automatic fine tuning circuit during reception of the television broadcasting signal of a predetermined channel, any distortion produced during reception is displayed on the video displaying screen and distortion in the voice signal is output.
Therefore, when a deviation is generated to the local oscillating section during viewing of a received television broadcasting signal of a predetermined channel, the television broadcasting signal cannot accurately be received. A user must turn off the electric power of the video receiving apparatus and then turn the power on again so that the television broadcasting signal of the predetermined channel is again adjusted by the automatic fine tuning circuit.
On the other hand, a television receiver provided with a PIP (Picture in Picture) function or a video receiving apparatus, such as a video cassette tape recorder, or a video receiving apparatus provided integrally with a television receiver and a video cassette tape recorder, includes two tuners.
That is, a television receiver provided with a PIP (Picture in Picture) function or a video receiving apparatus such as video cassette tape recorder receives a television broadcasting signal of a main picture by a main tuner, while a television broadcasting signal of PIP screen is received by a sub-tuner section. The television broadcasting signal of the received PIP screen displays two television broadcasting signals on one screen by inserting the television signal of the PIP screen into the television broadcasting signal of the main picture screen.
The video receiving apparatus provided integrally with the television receiver and the video cassette tape recorder both operate with two tuner sections when simultaneously viewing a television broadcasting signal received by a main tuner, and recording the television broadcasting signal received by sub-tuner section.
The video receiving apparatus provided with the two tuner sections uses only the main tuner section when the PIP function is not used or when simultaneous recording and viewing of television broadcasting signals is not being performed, and the sub-tuner section is not operated.
Therefore, the using range of the sub-tuner section is very limited, and it is inefficient.